What is the ideal weight/mass for a turntable?

I think the the issue of turntable mass is largely misunderstood. The more important issues are rigidity, damping and rotation stability. Mass has little if any direct impact on these key issues.

Rigidity is very important and it often requires a lot of mass to get it. A good milling machine weighs at least a half ton because it needs to be very rigid. The most cost effective way to get rigidity with a mill is to use lots of steel and iron. You can get the same rigidity with lighter
materials but it would just raise the cost with no benefit. With turntables there are a lot of ways to get rigidity and the most cost effective ways tend to also be heavy. So rigidity is the goal and mass is just a side effect. Heavy turntables will often have superior rigidity, but this is not always the case. For example a carbon fiber base will lighter and more rigid than one of the same dimensions made from aluminum.

Damping is also frequently misunderstood in it's relation to mass. Adding mass to a turntable (or any structure) can not by itself increase damping. The argument of high mass being more difficult to excite is bogus. A 150 pound chunk of granite rings like bell. It take no less energy to get 150 pounds of granite to ring than it does a wind chime. However, like rigidity, effective damping techniques are often also heavy. Sand and lead shot are very effective
and simple ways to damp resonance. It is not the mass that helps it's the damping. So once again high mass is just a side effect of some of the better methods for damping.

Rotational stability is certainly affected by mass but there is much more to the story than meets the eye. Mass mostly effects the frequency of speed perturbations but it does not make speed more constant in absolute terms. A heavy platter responds to a perturbation with a small speed variation but it distributes that variation over longer period of time. Conversely a light platter will have larger speed variations but the recovery will be quicker. Most, but not all, prefer the resulting sound of a heavy platter.

So in the end high mass turntables will often be superior to their lighter weight peers. But if so it will be because of better rigidity, damping and/or rotational stability, and not because they are "heavy".

For starters I think the "resonance diode" concept is far fetched. I cannot envision any theoretical basis for such a device, and simple cones do not behave as a "diode".

The principles behind resonance energy transfer are well understood and quite logical. But understanding how to apply those principles in real life situations is often very complex. That's where the art of audio design comes in.

I believe that elimination of vibrational energy is one of the most important parts of good turntable design. Vibrational energy cannot be removed but it can be converted into another form. Damping mechanisms of all types convert vibrational energy into heat. Damping and energy dissipation are synonymous terms. If a device does not convert movement to heat then by definition it is not a damper.

Another important principle is the conduction of vibrational energy. To dissipate the energy you often need to conduct it to a different place. That is where cones come in. They are often referred to as isolation devices but the opposite is true. Cones are very good at conducting energy and that is why they work well. Cones under a CD transport will conduct vibrational energy from inside the transport into the underlying shelf. Cones are effective when there is more internal vibrational energy than external. More often than not the worst vibrations are emanating from inside of a component. Yet common audiophile thinking is that components need to be isolated. If that were the case cones would make things worse.

Turntables in particular require energy dissipation. The stylus/record interface generates a remarkable amount of vibrational energy. To keep that energy from bouncing around and mucking up the sound it has to be dissipated (converted to heat) somewhere. In many cases the primary mechanism is the suspension. Even though suspension does provide some isolation I believe that the energy dissipation aspect is far more important. In fact effective isolation without energy dissipation would produce very poor results. Vibrations would have no place to go so they bounce around in the turntable causing trouble.

Most, but not all unsuspended turntables utilize some sort of internal damping to dissipate energy. We have found that loose lead shot is a very effective damping medium. Considerably better than constrained layer or any sort of
suspension. So the same principles are at work with both suspended and unsuspended turntables. The methods are different and the resulting sound is also different. However, I believe that the primary benefits of both are from energy dissipation and not isolation.

There is a common misconception that an unsuspended turntable is more susceptible to external vibration. In my experience there is little difference in absolute terms. Both types of turntable respond well to being placed on a rigid, stable platform. Both types of turntables can be negatively affected by footfalls and springy floors. Exactly how they are affected in a given situation may however vary widely. But as far as I can tell there is no inherent advantage for an unsuspended or suspended turntable. Better implementations will tend to be less affected. In many cases I think it is simply synergy. A particular design may work flawlessly with one stand, floor and room combination and perform poorly in another. Just read the forums. You will find diverse opinions and experiences not because people are biased or nuts, but rather the real life experiences understandably different.

While I do not think there is much difference in sensitivity to external vibration for suspended vs unsuspended turntables, I am not implying that they sound the same. I personally believe that a well implemented unsuspended turntables deliver more satisfying and accurate sound. But I certainly can understand why someone else may prefer the "suspended" sound.

"Also why not build a plinth from carbon-fibre....is this too difficult or expensive?"

We have done some experimenting with carbon fiber and it sounds very good. The texture and detail that we hear with dense hardwoods, but lacking some of the warmth and delicacy. A very good material but in our opinion not quite as good sounding as hardwoods and quite a bit more expensive.